1. The Field of the Invention
This invention relates to copolymers. More precisely, the invention disclosed herein relates to synthetic copolymers useful as hardeners for protein-containing emulsions such as gelatin emulsions employed in photographic processes.
2. Description of the Prior Art
Protein-containing emulsions, especially gelatin-containing emulsions, are known products of commerce routinely employed in the photographic art, for example, as light-sensitive silver halide emulsion layers. As those skilled in the art know, such layers encounter processing conditions which can cause swelling or degradation of the gelatin or which can otherwise adversely affect or impair the desired properties of the gelatin layer. In order to avoid or at least compensate for this problem, hardeners which can reduce swelling of the protein are included in emulsions used for forming such layers.
The term "hardeners" is not precisely defined in the art. However, as those skilled in the art will agree, the effect of including a hardener in a protein-containing emulsion is evidenced by a decrease in the swellability of the protein emulsion after coating, or an increase in the viscosity of the emulsion before coating. It is in this context that the term hardener is employed in the present invention, since in one aspect, the copolymers are employed as hardeners in a conventional sense, e.g., to reduce the swellability of protein-containing coatings or layers and protect the layers or coatings during processing conditions. However, in another aspect, the copolymers can be advantageously employed to selectively adjust the viscosity of protein-containing emulsions to a predetermined level so that the emulsion can be conveniently cast or formed as a coating or layer on a substrate.
In general, hardeners known to the art to be useful in gelatin layers are classified as follows:
1. Metallic salts such as those of chromium, aluminum or zirconium;
2. Aldehydes, or dialdehydes such as formaldehyde or mucochloric acid;
3. 1,2 or 1,4 diketones such as cyclohexane-1,2-dione;
4. Quinones; and
5. Higher molecular weight hardening agents such as oxidation products of starch and semiesters of maleic acids with polyvinyl alcohol or water-soluble bisulfite compounds of polyacrolein.
The use of the above-mentioned hardening agents has not met with the complete degree of success desired in all instances. For example, some of the hardeners are volatile or can otherwise migrate during processing and cause unwanted staining or discoloration. Others have a tendency to fog the light-sensitive emulsion layer or cause loss of speed. Moreover, some tend to increase the brittleness of the gelatin layer, especially during protracted storage. Accordingly, there is a need outstanding in the art for improved hardeners, especially those which are non-migratory in nature and which can provide improved mechanical properties and water resistance for protein-containing layers, especially at elevated temperatures.
Recently, in U.S. Pat. Nos. 3,382,077 and 3,396,029, non-migratory hardeners have been presented to the art. Essentially, the hardeners described in U.S. Pat. No. 3,382,077 are addition products or acrolein with high molecular weight hydroxy group-containing compounds such as polyvinyl alcohols, derivatives thereof and the like. The addition products contain free aldehyde groups attached to the polymeric chain and the groups can harden gelatin or like proteins.
A non-migratory type hardener is also described in U.S. Pat. No. 3,396,026. Essentially, the hardener described therein is a copolymer comprising acrolein and a comonomer containing carboxylic groups. The preferred hardeners are copolymers containing about 50 to 95 parts by weight acrolein and about 5 to 50 parts by weight of a copolymerizable carboxylic acid such as acrylic acid, methacrylic acid or maleic acid. Especially preferred are copolymers containing 80 to 95 parts by weight of acrolein and 5 to 20 parts by weight of acrylic acid and/or methacrylic acid.
The above-described non-migratory hardeners are employed in conventional manners. In other words, they are included in protein-containing emulsions which are subsequently cast in the form of films or coatings which manifest reduced swellability under processing conditions. However, the above-mentioned prior art does not discuss the use of such hardeners as a means for controlling or adjusting the viscosity of protein-containing emulsions prior to the casting or coating steps. As mentioned, one aspect of the present invention involves the use of the novel non-migratory hardeners described herein to selectively adjust the viscosity of protein-containing emulsions to predetermined levels.